The open field above every head — where the moon keeps her phases, the wanderers walk, and the year turns its wheel. The sky is the one calendar every hearth shares.
Old Norse sky (a cloud), from Proto-Germanic *skeujo-, related to a root meaning covering; borrowed into English to name the whole vault of heaven, a sense the older Old English heofon had held alone.
The word itself once reached no further than a single passing cloud, and only later widened to claim the whole vault standing behind it — the sky watched now is bigger than the word that first named it.